One class of liquid crystal devices, or cells, uses nematic phase liquid crystal materials having chiral or twisting additives (often referred to as "twisted nematic phase" materials) wherein the rod-like liquid crystal molecules, often referred to in the art as "directors", lie in different directions from one surface of the cell to the other. Such twisted material cell structures are often used in display systems, e.g., watch displays, calculator displays, and the like.
Another class of liquid crystal cells uses nematic phase materials which are free of chiral or twisting additives, which cells, while less widely used for display devices, can be useful if properly designed in systems which require control or stabilization of light sources, particularly modulation of monochromatic light sources, such as lasers, and in systems which are used to produce light retardance for use in, e.g., birefringent filters. In such cells, a liquid crystal material with dielectric and optical anisotropies is placed between parallel transparent plates having transparent electrodes. When no electrical voltage is placed across the electrodes of the plates the directors lie at some angle .THETA..sub.t to the plates substantially along the direction of micro-scratches or other alignment pattern which has been created on the surface of the plates. The pattern of scratches or an alignment grid pattern is generally termed the "alignment layer" of the cell. The alignment layer is highly anisotropic, having a preferred axis along the direction of the alignment pattern, such axis normally being referred to as the alignment layer axis. The angle .THETA..sub.t is generally called the "tilt" angle and is a function of the particular surface treatment used to form the alignment pattern.
The optical path length is different for light with a polarization axis parallel to the long axis of the directors than for light with a polarization axis orthogonal to such long axis. Because the liquid crystal molecules tend to align themselves along the alignment layer axis, the macroscopic optical index of refraction is different for light polarized parallel to the alignment layer axis than for light polarized orthogonally to it. Such a cell has a retardance equal to the optical index anisotropy multiplied by the cell thickness. When an electric field E is imposed thereon, by applying a suitable voltage across the transparent electrodes, the liquid crystal directors tend to align themselves in the direction of the electric field, which direction is normal to that of the alignment pattern. When the intensity of the electric field is sufficiently strong so that substantially all of the directors are so aligned, normally termed a "fully driven" state, there is essentially no optical path difference for the two polarization components and the retardance is substantially zero.
This variation of optical retardance between a non-driven and a fully driven state forms the basis for a number of light modulators. First, the cells can produce an electrically-variable optical path length for light polarized parallel to the alignment pattern. This operation is useful in a variety of systems including etalons. Second, for light polarized 45 degrees to the alignment pattern, the cells can perform a polarization rotation by acting as a variable waveplate.
Although the above-discussion considers only two operating states (fully driven and non-driven, i.e., fully relaxed) there is a continuous range of states between the fully driven and the fully relaxed states. In these intermediate states, or "partially driven" states, some of the liquid crystal directors lie against, or relatively near, each alignment layer (and are "relaxed") and some of the directors in the middle region of the cell are normally aligned (are "driven"). These intermediate retardance states are achieved by applying an electric field which is less intense than that required to achieve the fully driven state.
In general, the voltage required to produce a given retardance is dependent upon the cell dimensions, temperature, and the properties of the liquid crystal material. Further, the form of the dependence between the applied electric field strength and optical retardance is not linear and not easily predicted. Due to these considerations, as well as to inevitable variations which occur in the manufacturing process and in the materials themselves, devices which will exhibit a known optical retardance for a given applied electric field strength are difficult to produce without the costly and time-consuming task of "tuning" each cell individually to obtain a particular cell having the desired characteristics.
Further, the switching speeds of such cells which have been proposed are relatively slow. If the cells are constructed so that the alignment layer axes of the opposing plates are antiparallel, the cell relaxes to a stable state but exhibits slow response time (e.g., about 10-50 ms.) due to hydrodynamic effects. In order to achieve faster switching speeds the prior art has suggested some modifications to the structure of such cells. U.S. Pat. No. 4,582,396, issued on Apr. 15, 1986 to Bos et al., describes a device with a relatively fast switching response. The device is constructed by placing the opposing alignment grids parallel to one another. Such construction permits faster switching times (e.g., about 1-2 ms.) by removing the hydrodynamic vorticity which is normally present in cells having antiparallel alignment grids. However, such a cell is only metastable in the relaxed state and it will decay after about 10-100 ms. with unpredictable performance thereafter. Accordingly, the usefulness of such a cell is limited only to systems where it will be energized to the driven state every 10 ms. or more often.
U.S. Pat. No. 4,385,806, issued on May 31, 1983 to Ferguson, describes the manufacture of relatively thick cells having an unspecified alignment orientation. The patent discloses cells of 50 to 75 micron thickness, for example, which cells are driven with a relatively large DC bias voltage so as to place most, but not all, of the liquid crystal directors into the driven state. The response then to a relatively small AC signal superimposed upon the DC bias is very rapid. Such fast response is explained as due to the modulation of the orientation angle of those directors present at the interface between the substantially driven and the substantially relaxed directors. However, such a cell demonstrates several shortcomings.
First, because the cell is so thick, it produces substantial retardance during operation and, thus, it must be used with compensating retarders in order to operate near zero retardance, an operating point frequently sought.
Second, a liquid crystal cell which uses a large DC bias exhibits migration of ionic species in the crystal material. Various ionic species are always present in the material, arising from impurities in the liquid crystal medium as well as from impurities which leach into the cell during manufacture or use. Such ions migrate to the electrode having a polarity opposite to that of their ionic charge. Once such migration occurs, the net electric field is equal to the field imposed by the electrodes, plus the field due to the migrated ionic species. Since the distribution of ionic species across the cell surface is non-uniform, the resultant electric field is also non-uniform. Such field non-uniformity produces spatial non-uniformities in the cell retardance, rendering it of low optical quality.
Finally, the use of a large DC bias field placed across the liquid crystal cell electrolyses the liquid crystal molecules, greatly shortening the useful life of the cell.